Wireless connection is becoming a very important connectivity mechanism for lighting control systems due to its ease of deployment, testing, commissioning and use. However, there is no dedicated wireless band for the transmission of lighting control signals in the current lighting control system. The ISM (Industrial, Science and Medical) band at 2.4 GHz is often used by many commercial lighting applications due to its worldwide free license. Besides lighting applications, the 2.4 GHz ISM band is also used by many other applications, e.g. Internet connection, building automation, and personal local area network, which use WiFi™, Bluetooth™ or Zigbee™.
The 2.4 GHz ISM band may be used by various systems at the same time, therefore, when the wireless lighting control system occupies a certain channel of the 2.4 GHz band for wireless transmission of the lighting control signal, other applications, e.g. IEEE802.11 WLAN (Wireless Local Area Network), which occupy the same channel will interfere with the wireless transmission of the lighting control signal. The wireless lighting control system can dynamically switch to a new channel when detecting interferences; however, switching to a new channel may lead to interruption of the lighting control signal transmission, e.g. a temporary pause of the dynamic lighting pattern, thereby jeopardizing a users' visual experience.
Therefore, when the wireless lighting control system starts to transmit a lighting control signal, first a clean channel (i.e. a channel not occupied by other systems) should be selected for the wireless transmission of the lighting control signal. Nowadays, a common approach used to determine whether a channel is interfered or not is to estimate the average signal strength of the signals received on the channel within a predetermined duration. If the estimated average signal strength is lower than a predetermined threshold, then the channel is determined to be clean, and vice versa. However, this approach can only indicate that the channel is clean during the predetermined detection duration.
Taking IEEE802.11 WLAN as the source of interference by way of example, when the above approach is used by the wireless lighting control system to determine whether a channel is interfered, the average received signal strength estimated by the wireless lighting control system will be rather low if there is no or less data traffic over IEEE802.11 WLAN which occupies the channel during the detection duration, so that the channel will be mistakenly judged to be clean and the wireless lighting control system will start the wireless transmission of the lighting control signal on the channel. However, some time later, once there is data traffic over IEEE802.11 WLAN, the wireless transmission of the lighting control signal of the wireless lighting control system on the channel will be interfered.